Poly(trimethylene terephthalate) (PTT) is primarily a linear aromatic polyester which can be prepared from the condensation polymerization of 1,3-propanediol (PDO) and terephthalic acid (TPA). Processes for making this polymer have been known for some time. For example, a process for making PTT is described in U.S. Pat. No. 3,366,597. An atmospheric pressure process for making PTT is described in U.S. Pat. No. 5,599,900. A continuous all melt low pressure process for making PTT is described in U.S. Pat. No. 6,277,947. This continuous all melt low pressure process makes PTT with a high enough intrinsic viscosity so that solid state polymerization of the PTT is not required. In most prior processes, solid state polymerization was required in order to increase the intrinsic viscosity to an acceptable commercial value. Solid state polymerization is also advantageous because the final product is low in acrolein content but solid state polymerization has the disadvantage that the PTT pellets are friable.
It is known that the addition of a hindered phenol to polytrimethylene terephthalate can help reduce the amount of acrolein generated when the PTT comes into contact with air. To date, this is not considered true for phosphorus compounds because phosphorus compounds reduce the amount of acrolein only under an inert atmosphere when added during the polymerization process, and do not reduce the amount of acrolein formed when the polymer is heated under air. U.S. Pat. No. 6,093,786 describes a process for making PTT which has a reduced tendency to form acrolein when heated in air. This process involves the addition, to the polycondensation reaction, of a hindered phenol and an aromatic organophosphite containing a trivalent phosphorus group of the formula (ArO)wP, in which AR is an aryl group and w is an integer from 1 to 3.
Example 12 of U.S. Pat. No. 6,093,786 describes that using tris(2,4-di-t-butylphenyl)phosphite during polymerization to make PTT did lower the amount of acrolein found in the pellets. However, the results of that example shown in Table 3 (before the solid state polymerization step) show that the retention of the phosphite in the final polymer is poor, i.e., a maximum amount of 26 ppm phosphorus when starting with either 50 or 75 ppm of the phosphite. Example 20, Table 7, shows that a high amount of the phosphorus compound is contained in the PDO distillate which is the PDO which is recovered from the process for recycle.
Any of the phosphorus species which volatizes during the process of making PTT can contaminate the PDO distillates and this contamination can interfere with the purification of these distillates for recycle. Even the nonphosphorus parts of some additives can interfere with PDO purification. For example, IRGAFOS™ 168 additive generates volatile 2,4-di-t-butylphenol which complicates the fractional distillation of the recovered PDO. In a preferred embodiment of the process of U.S. Pat. No. 6,277,947, recovered excess PDO is directly recycled from vacuum spray loops without purification. The phosphorus compounds and their byproducts can interfere with the polymerization if recycled into the PTT process; for example, by reducing the activity and efficiency of the process catalysts. Furthermore, volatile phosphorus compounds, the compounds themselves or phosphorus-containing degradation products of the compounds, can interfere with the operation and efficiency of catalytic oxidizers used to destroy volatile by product streams from the process because phosphorus compounds poison the catalysts commonly used in such oxidizers.
Therefore, it can be seen that it would be useful to have a process, particularly an all-melt process without solid-state polymerization, for making PTT which utilizes phosphorus compounds to reduce the amount of acrolein and which are retained to a high degree in the final PTT product from the process.
U.S. Pat. No. 5,744,572 describes a process for the acceleration of the polycondensation of polyester, i.e. meaning polyethylene terephthalate (PET). From 30 to less than 500 ppm, typically from about 120 ppm to about 300 ppm, of a phosphorus compound which is a carboxy phosphonic acid is added prior to precondensation to accelerate the polycondensation of the polyethylene terephthalate. For the production of PTT, the addition of large amounts of the phosphorus compounds is a disadvantage because they tend to slow down the polycondensation rate which results in a polymer with a lower intrinsic viscosity over the same reaction time.